The Illitization Process in Deeply Buried Shales of the Gulf Coast Area

Clauer, Norbert

doi:10.1346/ccmn.1995.0430214

Cited by 5 publications

(2 citation statements)

References 15 publications

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“…Some investigators favor a DC mechanism for describing illitization , others favor an OR mechanism (Eberl et al 1990) and others favor an SST mechanism (Hurmiker et al 1986;Clauer et al 1995). Several studies emphasize that different mechanisms apply in different geologic environments, an SST-like mechanism describing illitization in bentonite or shale (Ahn and Peacor 1986a;Inoue et al 1990) and a DC or OR model describing illitization in hydrothermal settings (Yau et al 1987;Inoue et al 1988).…”

Section: Reaction Mechanisms For Smectite Illitizationmentioning

confidence: 99%

Comparison of Structural Models of Mixed-Layer Illite/Smectite and Reaction Mechanisms of Smectite Illitization

Altaner

1997

Clays and clay miner.

221

129

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Abstract--This paper compares mechanisms of the reaction of smectite to illite, in light of structural models for interstratified illite/smectite (US). The crystal structure of US has been described previously by a nonpolar and polar 2:1 layer model. In a nonpolar model, individual 2:1 layers are chemically homogeneous, whereas a polar model assumes a 2: l layer can have a smectite charge on one side and an illite charge on the other side. Several kinds of data support the polar model; however, more determinations of the negative charge of expandable sites in US are needed to confirm such a model.Assuming a polar 2:1 layer model for I/S, we compare the mineralogical and geochemical consequences of several reaction mechanisms for smectite illitization: 1) solid-state transformation (SST), 2) dissolution and crystallization (DC) and 3) Ostwald ripening (OR). Features of an SST model are the replacement of smectite interlayers by illite intedayers, resulting in gradual changes in interlayer ordering, polytype, chemical and isotopic composition and crystal size and shape. Several SST models are possible depending on the nature of the reaction site (framework cations, polyhedra or interlayers). In contrast, DC models allow for abrupt changes in the structure, composition and texture of I/S as illitization proceeds. Several DC models are possible depending on the nature of the rate-controlling step, for example, diffusional transport or surface reactions during crystal growth. The OR model represents the coarsening of a single mineral where the smallest crystals dissolve and nucleate onto existing larger crystals, allowing for evolution in the overgrowth but not in the template crystal.An SST mechanism, involving either reacting polyhedra or reacting interlayers, seems to best model illitization in rock-dominated systems such as bentonite. A DC mechanism seems to best model illitization in fluid-dominated systems such as sandstone and hydrothermal environments. Both DC and SST mechanisms can occur in shale. Differences in reaction mechanism may be related to permeability. An OR model poorly describes illitization because of the progressive mineralogical and chemical changes involved. For many geologic environments, it is important to consider alternate origins for US such as kaolinite illitization and detrital. Further work is needed to clarify the DC crystal growth process in terms of a structural model of I/S and to determine which specific SST or DC model best characterizes illitization in geologic systems.

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Section: Reaction Mechanisms For Smectite Illitizationmentioning

confidence: 99%

Comparison of Structural Models of Mixed-Layer Illite/Smectite and Reaction Mechanisms of Smectite Illitization

Altaner

1997

Clays and clay miner.

221

129

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“…48, No. 6 , 2000 Reaction mechanisms of smectite illitization from Ponza island, Italy 61 1 (Eberl et al, 1990), and still others favor an SST mechanism (Hunziker et al, 1986;Lindgreen et al, 1991;Clauer et al, 1995). Several studies emphasize that different mechanisms may apply in different geologic environments: an SST or local DC mechanism describing illitization in shale and bentonite (Inoue et al, 1990;Ahn and Peacor, 1986; Li et al, 1997) and a DC or Ostwald ripening mechanism in hydrothermal settings (Yau et al, 1987;Inoue et al, 1988;Christidis, 1995).…”

Section: Introductionmentioning

confidence: 99%

Reaction Mechanisms of Smectite Illitization Associated with Hydrothermal Alteration from Ponza Island, Italy

Ylagan

2000

Clays and clay miner.

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Abstract---A hydrothermally altered rhyolitic hyaloclastite from Ponza island, Italy, has four alteration zones with unique clay assemblages: (1) a non-pervasive argillic zone characterized by smectite; (2) a propylitic zone with interstratified illite-smectite (I-S) containing 10-85% illite (I); (3) a silicic zone composed of I-S with -->90% I and pure illite; and (4) a sericitic zone with I-S ranging from 66% I to pure illite. Atomic force microscopy reveals abrupt changes in particle morphology with illitization, including initial changes from anhedral plates to laths and then to eubedral plates and hexagonal plates. I-S particles progressively thicken with illitization and mean particle area (basal plane) remains constant from pure smectite to I-S with 80% I. However, particle area increases from 90 to 100% illite. Computer modeling of I-S structural forms indicates octahedral cation ordering progressively changes from cisvacant smectite to interstratified cis-and trans-vacant l-S, and then to trans-vacant illite. In addition, polytypes progressively change from 1M d to 1M, and then to 2MI illite. Electron-microprobe and X-ray fluoresence analyses show that I-S chemistry progressively changes during illitization, evolving toward a phengitic composition with --0.89 fixed interlayer K + per O~0(OH)2. Octahedral Mg 2+ shows little change with illitization, varying from 0.3 to 0.5 cations per O~0(OH)2. The layer charge of smectite is --0.38 equivalents per OI0(OH) 2.On the basis of abrupt changes in morphology and progressive changes in polytype and chemistry, smectite illitization on Ponza involved a dissolution and recrystallization mechanism with multiple stages of nucleation and crystal growth. In this multi-step model, temperature of alteration provided the major control for the layer composition, polytype, and morphology of I-S crystallites. Other factors that may play a secondary role include: K + availability, water-rock ratio, and permeability. Alternatively, the mechanism of I-S and illite formation at Ponza and other hydrothermal environments may occur by direct precipitation of I-S crystallites from rhyolite glass and may not involve progressive reactions of smectite precursors.

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